(4) 三角形的高一定，它的面积和底()。
① 成正比例
② 成反比例
③ 不成比例

3. 解比例。
$6:1.2 = x:2.4$
$\frac{2}{3}:\frac{5}{6} = \frac{1}{5}:x$
$\frac{x}{30} = \frac{0.5}{6}$
$\frac{3}{8}:x = \frac{5}{8}:\frac{7}{8}$

解比例
1. $6:1.2 = x:2.4$
解：$1.2x = 6 × 2.4$
$1.2x = 14.4$
$x = 14.4 ÷ 1.2$
$x = 12$
2. $\frac{2}{3}:\frac{5}{6} = \frac{1}{5}:x$
解：$\frac{2}{3}x = \frac{5}{6} × \frac{1}{5}$
$\frac{2}{3}x = \frac{1}{6}$
$x = \frac{1}{6} ÷ \frac{2}{3}$
$x = \frac{1}{6} × \frac{3}{2}$
$x = \frac{1}{4}$
3. $\frac{x}{30} = \frac{0.5}{6}$
解：$6x = 30 × 0.5$
$6x = 15$
$x = 15 ÷ 6$
$x = 2.5$
4. $\frac{3}{8}:x = \frac{5}{8}:\frac{7}{8}$
解：$\frac{5}{8}x = \frac{3}{8} × \frac{7}{8}$
$\frac{5}{8}x = \frac{21}{64}$
$x = \frac{21}{64} ÷ \frac{5}{8}$
$x = \frac{21}{64} × \frac{8}{5}$
$x = \frac{21}{40}$